EP3531156B1 - Adjusting a field distribution of an antenna assembly of a magnetic resonance system - Google Patents

Description

The present invention relates to a method for adjusting a field distribution of an antenna arrangement of a magnetic resonance system. The present invention relates in particular to a method for compensating for mechanical tolerances of an arrangement of a high-frequency antenna in a high-frequency screen of a magnetic resonance system. The present invention further relates to a magnetic resonance system with an antenna arrangement, a computer program product and an electronically readable data carrier, which contribute to the implementation of the method.

In magnetic resonance systems (MR systems), antenna arrangements can be used to generate a high-frequency field, a so-called B1 field. Such antenna arrangements can, for example, have a cylindrical or tubular structure which is arranged around a patient receiving opening of the magnetic resonance system. Examples of antenna arrangements, which are often also called body coils, are in the publications US 8362775 B2 , US 6781378 B2 , US 8072218 B2 and US 20170016969 A1 described.

The DE 10 2011 089448 A1 relates to a breast coil for a magnetic resonance tomography device for generating magnetic resonance images of female breasts, with a coil housing with a breast recess for receiving a breast and with a number of coil elements. At least one of the coil elements forms an HF correction coil element and for this purpose has a circuit arrangement to switch the HF correction coil element between an HF correction operating state and another operating state. The HF correction coil element is designed so that in the HF correction operating state it passively resonates with a B ₁ field emitted by a transmitting antenna arrangement of the magnetic resonance tomography device and influences the local B ₁ field distribution during a magnetic resonance recording.

The US 2013/021033 A1 relates to a system for adjusting a radio frequency (RF) magnetic field in an MR imaging unit using an RF transmitter coil for generating a radio frequency (RF) magnetic field and a plurality of RF receiver coils for receiving RF signals for magnetic resonance (MR) image data acquisition. An RF transmitter coil generates an RF magnetic field. An RF receiver coil receives an RF signal for MR image data acquisition and couples a magnetic field from the RF receiver coil to the RF transmitter coil for adaptively changing the RF magnetic field generated by the RF transmitter coil in response to an inhomogeneity in the RF magnetic field generated by the RF transmitter coil to reduce applying an RF pulse to the RF transmitter coil.

The WO 2007/124246A1 relates to a magnetic resonance system with a high-speed coil operating mode switch. A hybrid circuit for cooperatively coupling a radio frequency control signal to a quadrature coil is configurable in at least one of at least two coil modes. A time sequence of the at least two coil operating modes can be determined and used to compensate for B1 inhomogeneity.

The US 5,081,418 A relates to a method of adjusting a radio frequency coil for use in MR imaging applications. The method uses shunts with a reactance in parallel with a switch to connect portions of the coil to ground, thereby limiting resonant operation of the coil to a known orientation.

As an alternative to the above-mentioned whole-body coils, such an antenna arrangement can also be provided in conjunction with a local coil. The antenna arrangement can include an actual antenna, for example a so-called birdcage antenna or a transverse electromagnetic resonator (TEM antenna), and a radio frequency screen (HF screen) in order to provide a defined environment for field generation. For the antenna arrangement to work optimally, the interaction between the antenna and the HF shield is crucial, for example to achieve a symmetrical or homogeneous field distribution. Among other things, the distance between the antenna and the HF screen must be maintained as precisely as possible. However, practical implementations of such antenna arrangements are based on a mechanical separation of the antenna and HF shield, which means that adjustment elements are required to compensate for the existing tolerances.

In addition to mechanical adjustment elements, adjustable capacitors, so-called trimming capacitors, can be provided in order to compensate for these mechanical tolerances and to improve the field distribution, for example to homogenize it or to increase field symmetry. However, such adjustment work is very time-consuming and therefore cost-intensive. Furthermore, the homogeneous or symmetrical field distribution of the B1 field emitted by the antenna arrangement can also be influenced during operation of the magnetic resonance system, for example by the patient to be examined.

The object of the present invention is therefore to improve a field distribution of an antenna arrangement of a magnetic resonance system, for example under the conditions described above.

According to the invention, this object is achieved by a method for adjusting a field distribution of an antenna arrangement of a magnetic resonance system, a magnetic resonance system, a computer program product and an electronically readable data carrier solved according to the independent claims. The dependent claims define embodiments of the invention.

According to the present invention, a method for adjusting a field distribution of an antenna arrangement of a magnetic resonance system according to claim 1 is provided. The antenna arrangement is a cylindrical antenna arrangement for generating radio frequency signals, which has a cylindrical radio frequency shield around the antenna arrangement. A symmetry of the field distribution is impaired by a positional inaccuracy of the antenna arrangement in the magnetic resonance system. A symmetry of the field distribution is impaired by a positional inaccuracy of the antenna arrangement in relation to the high-frequency screen (HF screen). An impairment of the symmetry of the field distribution can include, for example, an impairment of the homogeneity of the field distribution inside the antenna arrangement. The antenna arrangement includes several antenna resonant circuit elements and several switching elements. A respective switching element of the plurality of switching elements is assigned to each antenna resonant circuit element of the plurality of antenna resonant circuit elements. A respective switching element is designed to functionally couple the associated antenna resonant circuit element to the antenna arrangement or to decouple it from the antenna arrangement depending on a switching state of the switching element. The combination of antenna resonant circuit element and associated switching element can therefore be viewed as a switchable antenna resonant circuit element.

The antenna arrangement can, for example, comprise an antenna with a birdcage structure. A birdcage structure consists of a number of equidistant, parallel longitudinal antenna rods arranged on a cylindrical surface. These longitudinal rods are connected to each other at high frequencies at the ends by antenna end rings. In this context, a high-frequency connection means that it is not necessarily a galvanic connection, but just one There is a transparent connection for high-frequency currents. The switchable antenna resonant circuit elements can be arranged, for example, between galvanically separated sections of the end rings. Alternatively, the switchable antenna resonant circuit elements can be arranged, for example, parallel to one of the longitudinal rods.

In the method, symmetry information of the field distribution in the antenna arrangement is measured. In particular, for example, symmetry information of the field distribution inside the antenna arrangement can be measured. In the case of an antenna arrangement with a birdcage structure, for example, symmetry information of the field distribution within the cylindrical surface, which is delimited by the longitudinal rods, can be measured. Based on the measured symmetry information, the multiple switching elements are automatically adjusted in such a way that the symmetry of the field distribution inside the antenna arrangement is increased.

By using switching elements, setting or adjusting the antenna arrangement can be automated in a simple manner. An automatic control can, for example, control different switching combinations for the multiple switching elements using a suitable method and thus optimize the field distribution. For example, a gradient descent method can be used or many or all possible switching combinations can be tried out and the best possible field distribution can be determined. The switchable antenna resonant circuit elements can thus compensate for mechanical tolerances that result from the interaction between the antenna and the high-frequency screen. Likewise, during operation of the magnetic resonance system, a patient's reaction to the field distribution within the antenna arrangement can be compensated for.

As described above, the positional inaccuracy of the antenna arrangement according to the invention includes a positional inaccuracy of an arrangement the antenna arrangement in relation to a high frequency shield.

This positional inaccuracy of the arrangement of the antenna arrangement in relation to the high-frequency screen can affect the symmetry of the field distribution inside the antenna arrangement. By adjusting the plurality of switching elements in such a way that the symmetry of the field distribution inside the antenna arrangement is increased, effects of the positioning are at least partially compensated for according to the invention.

In one embodiment, measuring the symmetry information includes measuring a resonance frequency of the antenna arrangement. Resonance frequencies and their decoupling can depend on the symmetry properties of the field distribution in the antenna arrangement. The multiple switching elements are adjusted based on the resonance frequency in such a way that a symmetry of the field distribution inside the antenna arrangement is increased, for example the multiple switching elements are adjusted so that one or more specific (desired) resonance frequencies and their decoupling occur. The resonance frequency of the antenna arrangement can be determined, for example, when feeding a high-frequency signal into the antenna arrangement and is therefore easily available as a measure of the symmetry or homogeneity of the field distribution.

In a further embodiment, measuring the symmetry information includes measuring a B1 field in the antenna arrangement. The multiple switching elements are adjusted based on the B1 field measurement in such a way that the symmetry of the field distribution inside the antenna arrangement is increased. The measurement of the B1 field can be carried out, for example, using appropriate sensors at suitable locations inside the antenna arrangement. This can, for example, ensure homogeneity or symmetry in various particularly relevant areas.

In yet another embodiment, measuring the symmetry information includes measuring a current distribution in the antenna arrangement. The multiple switching elements are adjusted based on the current distribution in such a way that the symmetry of the field distribution inside the antenna arrangement is increased. The current distribution in the antenna arrangement can be carried out, for example, with suitable current sensors, so-called pick-up coils. The current sensors can be assigned to the switching elements, for example. Alternatively or additionally, the current sensors can be assigned to individual elements of the end rings or the longitudinal rods of an antenna with a birdcage structure. Furthermore, the current sensors can be arranged in areas of galvanic isolation between individual elements of the end rings.

The current distribution represents an indicator of the homogeneity or symmetry of the field distribution of the high-frequency field generated by the antenna arrangement and is therefore suitable for controlling the switching elements.

An antenna resonant circuit element of the plurality of antenna resonant circuit elements can comprise, for example, a capacitor. The capacitor is coupled to the antenna arrangement by means of the associated switching element in order to change a capacitance of the antenna arrangement in order to increase the symmetry or homogeneity of the field distribution in the antenna arrangement. For example, a capacitor can be arranged in series with the associated switching element between two galvanically isolated elements of an end ring in order to increase the capacitive coupling between the galvanically isolated elements of the end ring depending on the switching state of the switching element. Depending on the asymmetry or inhomogeneity of the field distribution, such an increase in capacitive coupling can improve the symmetry or homogeneity of the field distribution.

Alternatively or additionally, an antenna resonant circuit element of the plurality of antenna resonant circuit elements can comprise, for example, an inductive element. The inductive element is connected to the antenna arrangement by means of the associated switching element coupled to change an inductance of the antenna arrangement to increase the symmetry or homogeneity of the field distribution. For example, a series circuit consisting of the inductive element and the associated switching element can be arranged parallel to one of the longitudinal rods of an antenna with a birdcage structure. With this arrangement, an inductance of the longitudinal rod can be increased depending on the switching state of the switching element. Such an increase in inductance can improve the symmetry or homogeneity of the field distribution.

In a further exemplary embodiment, the plurality of switching elements can be switched independently of one another. This makes it possible to precisely adjust the field distribution and thus compensate for positional inaccuracy or other asymmetries or inhomogeneities in the field distribution.

According to the present invention, a magnetic resonance system with an antenna arrangement according to claim 8 is further provided. According to the invention, a symmetry of a field distribution of the antenna arrangement is impaired by a positional inaccuracy of the antenna arrangement in the magnetic resonance system. The antenna arrangement includes a plurality of antenna resonant circuit elements, a plurality of switching elements and a control device which is coupled to the plurality of switching elements for controlling the switching elements. Each antenna resonant circuit element of the plurality of antenna resonant circuit elements is assigned one switching element of the plurality of switching elements. A respective switching element is designed to functionally couple the associated antenna resonant circuit element to the antenna arrangement depending on a switching state of the switching element. The control device is designed to measure symmetry information of the field distribution in the antenna arrangement and to automatically control the plurality of switching elements based on the measured symmetry information set in such a way that a Symmetry of the field distribution inside the antenna arrangement is increased.

The antenna arrangement is designed to carry out the method described above and therefore also includes the advantages described above in connection with the method.

The switching elements can include, for example, diodes whose switching state can be adjusted using a bias voltage. The preload can be adjusted by the control device. Alternatively or additionally, the switching elements can comprise transistors which are controlled by the control device.

The antenna arrangement can have a birdcage structure, for example. In an exemplary antenna arrangement, the antenna arrangement includes two end rings. Each of the end rings has a plurality of conductive surfaces isolated from one another in the circumferential direction. The several conductive surfaces are therefore galvanically isolated from one another. An antenna resonant circuit element of the plurality of antenna resonant circuit elements can, for example, comprise a capacitor. The capacitor is arranged between two conductive surfaces of one of the end rings. For example, the capacitor can be arranged together with its associated switching element between the two conductive surfaces in such a way that a capacitive coupling between the two conductive surfaces is increased by the capacitance of the capacitor when the switching element is closed. This allows the capacitive coupling between the conductive surfaces of the end rings to be changed.

The two conductive surfaces of one of the end rings can have a capacitive coupling to one another, for example due to their geometric arrangement to one another or due to capacitive elements arranged between them. The switchable capacitor can have a capacity in the range of 1% to 20% of the capacitive coupling of the two conductive surfaces exhibit. In particular, the switchable capacitor can have a capacity in the range of 5% to 10% of the capacitive coupling of the two conductive surfaces. A change in the capacitive coupling between the two conductive surfaces in the range of, for example, 1% to 20% is generally sufficient to compensate for positioning tolerances of the antenna arrangement in relation to the high-frequency screen in a magnetic resonance system.

In a further exemplary antenna arrangement, the antenna arrangement comprises a plurality of longitudinal rods. An antenna resonant circuit element of the plurality of antenna resonant circuit elements can include, for example, an inductive element. The inductive element is arranged parallel to one of the plurality of longitudinal bars in the longitudinal direction. For example, the inductive element together with its associated switching element can be arranged parallel to one of the plurality of longitudinal bars in such a way that an inductance of the longitudinal bar is increased.

The inductive element can, for example, have an inductance in the range of 1% to 20% of the inductance of the longitudinal rod. In particular, the inductive element can have an inductance in the range of 5% to 10% of the inductance of the longitudinal rod. As a result, the inductance of the longitudinal rod can be increased by the inductance of the inductive element depending on a switching state of the switching element. Such a change in the inductance of a longitudinal rod in the range of, for example, 1% to 20% is generally sufficient to compensate for positioning tolerances of the antenna arrangement in relation to the high-frequency screen in a magnetic resonance system.

The magnetic resonance system can also, for example, have a high-frequency control device for controlling the antenna arrangement, a gradient control unit, an image sequence control and a computing unit that is designed to acquire MR data of a predetermined volume section within an examination object. According to the invention, the symmetry of the field distribution is impaired by a positional inaccuracy of the antenna arrangement in the magnetic resonance system. The positional inaccuracy of the antenna arrangement includes a positional inaccuracy of an arrangement of the antenna arrangement with respect to a high-frequency screen, wherein adjusting the plurality of switching elements such that a symmetry of the field distribution inside the antenna arrangement is increased at least partially compensates for the positional inaccuracy.

The magnetic resonance system is designed to carry out the method described above and therefore also includes the advantages described above in connection with the method.

The present invention further relates to a computer program product according to claim 14.

With this computer program product, all or various previously described embodiments of the method according to the invention can be carried out when the computer program product is running in the control device. The computer program product may require program resources, e.g. libraries and auxiliary functions, in order to implement the corresponding embodiments of the method. In other words, the claim directed to the computer program product is intended to protect, in particular, software with which one of the above-described embodiments of the method according to the invention can be carried out or which carries out this embodiment. The software can be a source code (e.g. C++) that still needs to be compiled and bound or that only needs to be interpreted, or it can be an executable software code that only needs to be loaded into the corresponding computing unit for execution.

Finally, the present invention relates to an electronically readable data carrier according to claim 15, e.g. a DVD, a magnetic tape, a hard drive or a USB stick, on which electronically readable control information, in particular software (see above), is stored. If this control information (software) is read from the data carrier and stored in a control device or computing unit of a magnetic resonance system, all embodiments according to the invention of the method described above can be carried out.

• FIG 1 stellt schematisch eine Magnetresonanzanlage gemäß einer Ausführungsform der Erfindung dar.
• FIG 2 zeigt schematisch eine Antennenanordnung gemäß einer Ausführungsform der vorliegenden Erfindung.
• FIG 3 zeigt schematisch einen Ausschnitt einer Antennenanordnung gemäß einer Ausführungsform der vorliegenden Erfindung.
• FIG 4 zeigt schematisch einen Ausschnitt einer Antennenanordnung gemäß einer weiteren Ausführungsform der vorliegenden Erfindung.
• FIG 5 zeigt schematisch einen Ausschnitt einer Antennenanordnung gemäß einer weiteren Ausführungsform der vorliegenden Erfindung.
• FIG 6 zeigt schematisch ein Schaltelement mit einer Ansteuerung gemäß einer Ausführungsform der vorliegenden Erfindung.
• FIG 7 zeigt schematisch ein Ablaufdiagramm mit Verfahrensschritten eines Verfahrens gemäß einer Ausführungsform der vorliegenden Erfindung.

The present invention is described in detail below using embodiments of the invention with reference to the figures. The same reference numbers in the different figures designate the same or similar components.

• FIG 1 schematically represents a magnetic resonance system according to an embodiment of the invention.
• FIG 2 shows schematically an antenna arrangement according to an embodiment of the present invention.
• FIG 3 shows schematically a section of an antenna arrangement according to an embodiment of the present invention.
• FIG 4 shows schematically a section of an antenna arrangement according to a further embodiment of the present invention.
• FIG 5 shows schematically a section of an antenna arrangement according to a further embodiment of the present invention.
• FIG 6 shows schematically a switching element with a control according to an embodiment of the present invention.
• FIG 7 schematically shows a flowchart with process steps of a method according to an embodiment of the present invention.

FIG 1 schematically represents a magnetic resonance system 10 (MR system). The magnetic resonance system 10 has a magnet 11 for generating a polarization field B0. An examination subject 13 arranged on a couch 12 can be moved into a cylindrical opening in the magnet 11 in order to record location-coded magnetic resonance signals or MR data from the examination subject 13. A cylindrical antenna arrangement 50 for generating high-frequency signals, in particular high-frequency pulses (HF pulses), is also provided around the cylindrical opening. In order to provide a defined environment for field generation by the antenna arrangement 50, a cylindrical high-frequency shield 40 is also provided around the antenna arrangement 50. By radiating high-frequency pulses with the Antenna arrangement 50 and switching of magnetic field gradients, the magnetization generated by the polarization field B0 can be deflected from the equilibrium position and spatially coded, and the resulting magnetization is detected by receiving coils. How magnetic resonance images (MR images) can be generated by irradiating the RF pulses and switching magnetic field gradients in various combinations and sequences is fundamentally known to those skilled in the art and will not be explained in more detail here.

The magnetic resonance system 10 also has a controller 20 that can be used to control the magnetic resonance system 10. The controller 20 has a gradient control unit 15 for controlling and switching the necessary magnetic field gradients. A high-frequency control device (HF control device) 14 is provided for controlling the antenna arrangement 50 and generating the HF pulses to deflect the magnetization. An image sequence control 16 controls the sequence of the magnetic field gradients and HF pulses and thus indirectly the gradient control unit 15 and the HF control device 14. An operator can control the magnetic resonance system 10 via an input unit 17 and MR images and others necessary for control can be displayed on a display unit 18 information is displayed. A computing unit 19 with at least one processor unit (not shown) is provided for controlling the various units in the controller 20 and for carrying out computing operations. Furthermore, a memory unit 21 is provided in which, for example, program modules or programs can be stored, which, when executed by the computing unit 19 or its processor unit, can control the process of the magnetic resonance system 10. The computing unit 19 is designed to calculate the MR images from the acquired magnetic resonance signals.

FIG 2 shows an exemplary structure of the antenna arrangement 50 in the form of a three-dimensional wire model. The antenna arrangement 50 of FIG 2 has a so-called birdcage structure (Birdcage structure). Such a birdcage structure consists of a number of equidistant, parallel longitudinal antenna rods 51 arranged on a cylindrical surface FIG 2 the antenna arrangement 50 has eight longitudinal rods 51. However, the antenna arrangement 50 can have any number of longitudinal antenna rods 51, for example six or more than eight, for example twelve or 16. These longitudinal rods 51 are each connected to one another at high frequencies at the ends by antenna end rings 52, 53. “High-frequency connected” in this context means that there is not necessarily a galvanic connection, but rather just a connection between the longitudinal rods that is permeable to high-frequency currents. For example, there are resonance capacitors 56 in the antenna end rings 52,53 between two connection points 54,55 of adjacent longitudinal antenna rods 51. The connection points 54,55 can be formed, for example, by conductive surfaces of the respective end ring 52,53. The resonance capacitors 56 can be formed, for example, by opposing surfaces of these conductive surfaces 54,55. Alternatively or additionally, resonance capacitors 56 can be arranged as discrete components between the conductive surfaces. The longitudinal rods 51 each have a corresponding inductance due to their physical expansion in the longitudinal direction.

In the in FIG 2 In the example shown, the end rings 52.53 are each round. Alternatively, the end rings 52, 53 can also consist of straight sections running between two longitudinal antenna rods 51.

The antenna arrangement 50 is connected to the high-frequency control device 14 via connecting lines 57, 58. The connecting lines 57, 58 are each connected to adjacent connection points next to a resonance capacitor 56. These connecting lines 57, 58 not only feed in the high-frequency pulses during transmission, but also also a tap of the captured magnetic resonance signals in reception mode.

For optimal performance of the antenna arrangement 50, a precise arrangement or precise interaction of the antenna arrangement 50 and the high-frequency screen 40 is crucial. Among other things, a precise arrangement of the antenna arrangement 50 in the high-frequency screen 40, for example a uniform distance between the antenna arrangement 50 and the high-frequency screen 40, is crucial for the homogeneity or symmetry of a high-frequency field generated by the antenna arrangement 50. Furthermore, one or more resonance frequencies and their decoupling of the antenna arrangement 50 can be influenced by the arrangement of the antenna arrangement 50 in the high-frequency shield 40. The performance of the antenna arrangement 50 can therefore be impaired by mechanical tolerances when installing the antenna arrangement 50. In addition, the performance of the antenna arrangement 50 during operation of the magnetic resonance system 10 can be impaired by, for example, arranging the patient 13 within the antenna arrangement 50, since a field distribution of the field generated by the antenna arrangement 50 can be influenced by the patient 13.

In particular, in order to compensate for the mechanical tolerances, the antenna arrangement 50 has switchable antenna resonant circuit elements.

FIG 3 shows a section of the antenna arrangement 50 FIG 2 . FIG 3 shows only some of the connection points 54,55 of the end rings 52,53 and only one longitudinal antenna rod 51 of the several longitudinal antenna rods. In the example of the FIG 3 Two resonance capacitors 56 are shown between two adjacent connection points 54, 55. The number of these resonance capacitors 56 between two adjacent connection points 54,55 is arbitrary. As previously described, the capacitance of these resonant capacitors 56 can also for example, generated by the connection points 54,55 themselves. The connection points 54,55 can, as in FIG 3 shown, include conductive surfaces which lie opposite each other on one side and can thus form a capacitance. The conductive surfaces 54, 55 are conductively connected to the longitudinal rod 51, whereby the longitudinal rod 51 can in turn be designed as a conductive surface.

Switchable antenna resonant circuit elements are available in the FIG 3 Combinations of a switching element and a capacitor are shown. For example, a switching element 59 is provided in series with a capacitor 60 as a switchable antenna resonant circuit element between the conductive surface 54 and the conductive surface 55. On the other side of the conductive surface 55, a series connection of a switching element 62 and a capacitor 63 is provided as a switchable antenna resonant circuit element to the conductive surface adjacent there. Corresponding switchable antenna resonant circuit elements are also provided on the end ring 53 between the conductive surfaces. When the switching element 59 is closed, the capacitance of the capacitor 60 also has an effect on the capacitive coupling between the conductive surface 54 and the conductive surface 55. The switching elements 59,62 can be controlled by the control device 14. For this purpose, a control line 61 is provided between the control device 14 and the switching element 59 and a control line 64 is provided between the control device 14 and the switching element 62.

With the help of the switchable antenna resonant circuit elements, the field distribution inside the antenna arrangement 50 can be adjusted in order, for example, to increase the symmetry or homogeneity of the field distribution and thus to compensate for mechanical tolerances in the positioning of the antenna arrangement 50 in the high-frequency shield 40. A satisfactory adjustment of the field distribution inside the antenna arrangement 50 can be achieved with just a few switchable antenna resonant circuit elements. For example, can Only two to four switchable antenna resonant circuit elements can be provided in each of the end rings 52, 53. Of course, a respective switchable antenna resonant circuit element can be provided in each of the end rings 52, 53 between each of the connection points 54, 55.

FIG 4 shows another example of a switchable antenna resonant circuit element. The switchable antenna resonant circuit element in FIG 4 comprises a combination of a switching element 65 and an inductive element 66. The inductive element 66 can, for example, be a conductor track or a flat conductor parallel to the longitudinal rod 51. When the switching element 65 is closed, the inductance of the inductive element 66 also has an effect on the inductance of the longitudinal antenna rod 51. The switching element 65 can be controlled by the control device 14. For this purpose, a control line 67 is provided between the control device 14 and the switching element 65. Several of these switchable antenna resonant circuit elements can also be provided in a longitudinal antenna rod 51.

With the switchable antenna resonant circuit elements in the longitudinal antenna rod 51, the field distribution inside the antenna arrangement 50 can be adjusted in order, for example, to increase the symmetry or homogeneity of the field distribution and thus to compensate for mechanical tolerances in the positioning of the antenna arrangement 50 in the high-frequency screen 40. A satisfactory adjustment of the field distribution inside the antenna arrangement 50 can be achieved with just a few switchable antenna resonant circuit elements. For example, a respective switchable antenna resonant circuit element can be provided in only a few of the longitudinal antenna rods 51, for example only in two or four of the longitudinal antenna rods 51. Of course, a corresponding switchable antenna resonant circuit element can be provided in each of the longitudinal rods 51.

The antenna resonant circuit elements between the conductive surfaces 54, 55 and in the longitudinal antenna rods 51 can be controlled independently of one another by the control device 14.

With the previously described switchable antenna resonant circuit elements in the antenna arrangement 50, for example, mechanical tolerances that result from the interaction between the high-frequency screen 40 and the antenna arrangement 50 can be compensated for. These switchable antenna resonant circuit elements can be used to adjust resonance frequencies and their decoupling. The resonance frequencies and their decoupling can be measured, for example, with the high-frequency control device 14 when controlling the antenna arrangement 50 and the switchable antenna resonant circuit elements can be controlled accordingly in order to achieve suitable target values for the resonance frequencies and their decoupling. The switchable antenna resonant circuit elements can be controlled, for example, using an optimization method, for example using a gradient descent method, in order to set a desired resonance frequency and their decoupling. Alternatively, all possible switching combinations of the switchable antenna resonant circuit elements can be tried out and the most suitable switching combination for operation can be selected.

In addition to the resonance frequencies and the decoupling, a current distribution in the antenna arrangement 50 can also be used as an optimization criterion. For this purpose, a current sensor can be assigned to each switching element. FIG 5 shows such an arrangement in which the switching elements in the end ring 53 are each assigned a corresponding current sensor 72,73. Alternatively or additionally, current sensors can also be provided which measure a current in the connection points 54, 55. In FIG 5 For example, current sensors 69-71 for measuring respective currents in the connection points 54, 55 in the end ring 53 are shown. The current sensors 69-73 can be connected to the high frequency control device 14 be coupled. The current sensors 69-73 can each include a so-called pick-up coil. Taking into account the information from the current sensors 69-73, the switchable antenna resonant circuit elements can be adjusted so that a homogeneous current distribution and thus also a homogeneous field distribution is achieved. The effects of asymmetries during installation can be at least partially compensated for. For example, if the antenna arrangement 50 is not placed centrally in the radio frequency shield 40, this can lead to an asymmetry in the B1 field in the transverse direction. This asymmetry can be compensated for by suitable control of the switchable antenna resonant circuit elements. It is clear that a corresponding current sensor does not have to be provided in the area of every switchable antenna resonant circuit element and every connection point in order to achieve a more homogeneous current distribution. Even with a smaller number of current sensors, asymmetries and inhomogeneities can be at least partially detected and thus compensated for.

Even during operation of the magnetic resonance system 10, that is, for example, when a patient 13 is moved into the magnetic resonance system 10, the switchable antenna resonant circuit elements can be controlled by the high-frequency control device 14 in order, for example, to compensate for repercussions of the patient 13 on the current distribution in the antenna arrangement 50. For this purpose, for example, the current sensors described above can be used. Alternatively or additionally, in addition to the current sensors, sensors for measuring the B1 field can also be used at various points to evaluate the B1 field and adjust the switchable antenna resonant circuit elements. For example, the B1 field can be measured at certain points with permanently installed sensors or with portable sensors and a so-called B1 map can be generated. Based on this B1 map, the B1 field distribution can be assessed by the high-frequency control device 14 and the switchable antenna resonant circuit elements be controlled accordingly to improve the B1 field.

In summary, with the help of the switchable antenna resonant circuit elements, mechanical tolerances and their influence on the tuning of the antenna arrangement 50 can be compensated automatically and cost-effectively.

FIG 6 shows schematically an example of a switching element, for example the switching element 59. In the in FIG 6 In the example shown, the switching element includes a diode. The diode can in particular comprise a so-called PIN diode. Alternatively or additionally, other switchable semiconductor elements can be used as switching elements, for example transistors.

The switching elements 59, 62, 65 in the end rings 52, 53 and the longitudinal antenna rods 51 can be switched on and off separately from each other via corresponding control lines. In FIG 6 On the left, the control device 14 applies a negative voltage compared to the cathode to the anode of the diode via the control lines 61. As a result, the diode blocks and the switching element 59 is not permeable to high-frequency currents, the switching element 59 is therefore "open". In FIG 6 On the right, a positive voltage relative to the cathode is applied by the control device 14 via the control lines 61 to the anode of the diode. As a result, the diode conducts and the switching element 59 is permeable to high-frequency currents, the switching element 59 is therefore "closed".

In FIG 7 a flowchart of a method with steps 101 and 102 is shown. The method can be carried out, for example, using the high-frequency control device 14.

In step 101, symmetry information of the field distribution in the antenna arrangement 50 is measured. The symmetry information can, for example, be a measure of homogeneity or indicate symmetry of a B1 field generated by the antenna arrangement 50. To determine the symmetry information, for example, a current distribution in the antenna arrangement 50 can be measured, resonance frequencies and their decoupling when feeding high-frequency signals into the antenna arrangement 50 can be measured, or the B1 field generated by the antenna arrangement 50 can be measured at different positions. In step 102, the multiple switching elements 59, 62, 65 in the end rings 52, 53 and in the longitudinal antenna rods 51 are automatically adjusted based on the measured symmetry information in such a way that the symmetry of the field distribution inside the antenna arrangement 50 is increased. The method steps 101 and 102 can be carried out interactively several times in order to optimize the symmetry of the field distribution inside the antenna arrangement 50.

Claims (15)

1. Method for adjusting a field distribution of an antenna arrangement of a magnetic resonance system, wherein the antenna arrangement (50) is a cylinder-shaped antenna arrangement (50) for generating radio-frequency signals, which has a cylinder-shaped radio-frequency screen (40) around the antenna arrangement (50), wherein the antenna arrangement (10) comprises multiple oscillating circuit antenna elements (60, 63, 66) and multiple switching elements (59, 62, 65), wherein each oscillating circuit antenna element out of the multiple oscillating circuit antenna elements (60, 63, 66) is in each case assigned a switching element of the multiple switching elements (59, 62, 65), wherein a respective switching element is embodied to couple the assigned oscillating circuit antenna element operatively to the antenna arrangement (50) in dependence on a switching status of the switching element, wherein the method comprises:

   - measuring (101) symmetry information on the field distribution in the antenna arrangement (50), and

   - automatically adjusting (101) the multiple switching elements (59, 62, 65) using the measured symmetry information such that a symmetry of the field distribution in the interior of the antenna arrangement (50) is increased, characterised in that the symmetry of the field distribution is negatively impacted by a positional inaccuracy of the antenna arrangement (50) in the magnetic resonance system (10), wherein

   the positional inaccuracy of the antenna arrangement (50) comprises a positional inaccuracy of an arrangement of the antenna arrangement (50) relative to the radio-frequency screen (40), wherein the adjustment of the multiple switching elements (59, 62, 65) such that a symmetry of the field distribution in the interior of the antenna arrangement (50) is increased at least partially compensates the positional inaccuracy.
2. Method according to claim 1, wherein the measurement of the symmetry information comprises a measurement of a resonant frequency of the antenna arrangement (50), wherein the multiple switching elements (59, 62, 65) are adjusted using the resonant frequency such that a symmetry of the field distribution in the interior of the antenna arrangement (50) is increased.
3. Method according to one of the preceding claims, wherein the measurement of the symmetry information comprises a measurement of a B1 field in the antenna arrangement (50), wherein the multiple switching elements (59, 62, 65) are adjusted using the B1 field measurement such that a symmetry of the field distribution in the interior of the antenna arrangement (50) is increased.
4. Method according to one of the preceding claims, wherein the measurement of the symmetry information comprises a measurement of a current distribution in the antenna arrangement (50), wherein the multiple switching elements (59, 62, 65) are adjusted using the current distribution such that a symmetry of the field distribution in the interior of the antenna arrangement (50) is increased.
5. Method according to one of the preceding claims, wherein one oscillating circuit antenna element out of the multiple oscillating circuit antenna elements (60, 63, 66) comprises a capacitor (60, 63), wherein the method further comprises:

   - coupling the capacitor (60, 63) to the antenna arrangement (50) by means of the assigned switching element in order to change a capacitance of the antenna arrangement (50) in order to increase the symmetry of the field distribution in the antenna arrangement (50).
6. Method according to one of the preceding claims, wherein one oscillating circuit antenna element out of the multiple oscillating circuit antenna elements (60, 63, 66) comprises an inductive element (66), wherein the method further comprises:

   - coupling the inductive element (66) by means of the assigned switching element to the antenna arrangement (50) in order to change an inductance of the antenna arrangement (50) in order to increase the symmetry of the field distribution in the antenna arrangement (50).
7. Method according to one of the preceding claims, wherein the multiple switching elements (59, 62, 65) can be switched independently of one another.
8. Magnetic resonance system with an antenna arrangement (50), wherein the antenna arrangement (50) has a cylinder-shaped antenna arrangement (50) for generating radio-frequency signals, which has a cylinder-shaped radio-frequency screen (40) around the antenna arrangement (50), wherein the antenna arrangement (50) comprises:

   - multiple oscillating circuit antenna elements (60, 63, 66),

   - multiple switching elements (59, 62, 65), wherein each oscillating circuit antenna element out of the multiple oscillating circuit antenna elements (60, 63, 66) is in each case assigned a switching element of the multiple switching elements (59, 62, 65), wherein a respective switching element is embodied to couple the assigned oscillating circuit antenna element operatively to the antenna arrangement (50) in dependence on a switching status of the switching element, and

   - a control apparatus (14) embodied to carry out the following steps:

   - measuring (101) symmetry information on the field distribution in the antenna arrangement (50), and

   - automatically adjusting the multiple switching elements (59, 62, 65) using the measured symmetry information such that a symmetry of the field distribution in the interior of the antenna arrangement (50) is increased, characterised in that the symmetry of the field distribution is negatively impacted by a positional inaccuracy of the antenna arrangement (50) in the magnetic resonance system (10),

   wherein the positional inaccuracy of the antenna arrangement (50) comprises a positional inaccuracy of an arrangement of the antenna arrangement (50) in respect of the radio-frequency screen (40), wherein the adjustment of the multiple switching elements (59, 62, 65) such that a symmetry of the field distribution in the interior of the antenna arrangement (50) is increased at least partially compensates the positional inaccuracy.
9. Magnetic resonance system according to claim 8, wherein the antenna arrangement (50) is embodied to carry out the method according to one of claims 2-7.
10. Magnetic resonance system according to claim 8 or claim 9, wherein the antenna arrangement (50) comprises two ferrules (52, 53), wherein each of the ferrules (52, 53) has in the circumferential direction multiple conductive surfaces (54, 55) that are insulated from one another, wherein one oscillating circuit antenna element out of the multiple oscillating circuit antenna elements (60, 63, 66) comprises a capacitor (60, 63), wherein the capacitor (60, 63) is arranged between two conductive surfaces (54, 55) of one of the ferrules (52, 53).
11. Magnetic resonance system according to claim 10, wherein the two conductive surfaces (54, 55) of the one of the ferrules (52, 53) have a capacitive coupling to one another, wherein the capacitor (60, 63) has a capacitance in the range of 1% to 20%, in particular in the range of 5% to 10%, of the capacitive coupling of the two conductive surfaces (54, 55).
12. Magnetic resonance system according to one of claims 8-11, wherein the antenna arrangement (50) comprises multiple longitudinal rods (51), wherein one oscillating circuit antenna element out of the multiple oscillating circuit antenna elements (60, 63, 66) comprises an inductive element (66), wherein the inductive element (66) is arranged parallel to one out of the multiple longitudinal rods (51) in the longitudinal direction.
13. Magnetic resonance system according to claim 12, wherein the one out of the multiple longitudinal rods (51) has an inductance, wherein the inductive element (66) has an inductance in the range of 1% to 20%, in particular in the range of 5% to 10%, of the inductance of the longitudinal rod (51) .
14. Computer program product, which comprises a program and can be loaded directly into a memory (21) of a programmable controller (20) of a magnetic resonance system (10) according to claim 8 to 13, with program means for carrying out all the steps of the method according to one of claims 1-7 when the program is executed in the controller (20) of the magnetic resonance system (10).
15. Electronically readable data medium with electronically readable control information stored thereupon, which is embodied to carry out the method according to one of claims 1-7 when the data medium is used in a controller (20) of a magnetic resonance system (10) according to one of claims 8 to 13.

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